I. Field of the Invention:
This invention relates to a substrate coated with multiple thick films wherein conductive layers are all prepared from copper-base conductor paste.
II. Description of the Prior Art:
As is well known, a hybrid integrated circuit has come to be more widely accepted in order to reduce the weight and size of electronic appliances. Such a composite integrated circuit is constructed by soldering semiconductor elements and small electronic parts to a thick film substrate which is constructed by printing thick film conductive materials and resistive materials on an insulated substrate.
The conventional method of manufacturing such a thick film substrate comprises the steps of providing a conductive layer on an insulated substrate prepared from ceramic material. The conductive layer is formed by screen printing a conductive paste containing, for example, silver-palladium (Ag/Pd) powders and then firing the printed pattern. Later, a resistive paste containing, for example, ruthenium oxide (RuO.sub.2) powder and glass frit is screen printed and then the mass is fired to provide a resistor. Last, the junction parts between the conductive layer and resistive material are radiated by YAG laser beams to trim the resistor value. To realize practical high density integration, the common practice is to build conductive layers in a multiple form with an insulation layer interposed between the adjacent conductive layers.
However, the above-mentioned conventional conductive paste of silver-palladium base has the drawbacks that the impedance is as high as 20-50 m.OMEGA. per unit area; silver migration caused by absorption of moisture tends to deteriorate electric insulation and consequently reduce reliability; and the involvement of precious metal undesirably raises the cost of the final product. Recently, therefore, it has been proposed to apply a copper-base conductive paste. This copper-base conductive paste has to be fired in an atmosphere of nitrogen gas in order to prevent the deterioration of the quality of the paste resulting from the oxidation of copper. However, the proposed copper-base conductive paste offers the advantage that it is possible to produce a conductor having an impedance as low as 2-5 m.OMEGA. per unit area at a low cost.
The aforementioned steps of producing a copper base conductive paste involves firing carried out entirely in an atmosphere of nitrogen in order to avoid the oxidation of the copper base conductive layer. To date, no material has been developed which enables a resistor having a resistance for practical application to be produced in an atmosphere of nitrogen with a high reproducibility. Therefore, the conventional process of manufacturing a substrate coated with multiple thick films involving conductive layers comprises the steps shown in FIG. 1(a) of printing a conductive paste of a silver-palladium base on an insulated substrate 11 prepared from a ceramic material such as alumina, and firing the mass in the air at a high temperature of about 850.degree. to 900.degree. C. to provide the underlying conductive layer 12. Then as shown in FIG. 1(b), an insulation layer 13 prepared from an insulation paste of a high temperature melting point crystalline glass dielectric base is printed on an area defined between the underlying conductive layer 12 and the later described overlying conductive layer intersecting the underlying layer 12. Then as shown in FIG. 1(c), resistor 14 prepared from ruthenium oxide base resistive paste is printed. The above-mentioned insulation paste and resistive paste are fired at the same time in the air at a high temperature about 850.degree. to 900.degree. C. to provide insulation layer 13 and resistor 14. Later as shown in FIG. 1(d), a copper-base conductive paste is printed on the insulation layer 13 and contacting with resistor 14. The whole mass is fired at a temperature of about 600.degree. to 650.degree. C. in an atmosphere of nitrogen to provide the overlying conductor 15. Last, laser beam trimming is applied to the resistor 14 to obtain the required resistance. Throughout the above-mentioned process the copper base paste is fired at the relatively low temperature of about 600.degree. to 650.degree. C. in order to prevent the resistance of the previously fired resistor from significantly changing during the firing of the copper base paste.
Namely, to date, resistive paste, conductive paste of silver-palladium base, and insulative paste have all been fired in the air before the conductive paste of copper base is printed. Afterward, the conductive paste of a copper base is fired at a low temperature in an atmosphere of nitrogen gas in order to prevent the oxidation of copper and to suppress changes in the resistance of resistor 14.
However, the above-mentioned conventional process has the drawbacks that the application of a silver-palladium paste as an underlying conductor leads to the high impedance of the conductor, and the deterioration of insulation caused by the migration of silver which tends to take place when the subject multiple substrate is used under the condition of high temperature and humidity. Therefore, the underlying conductor has been applicable only as a short jumper line. Further, a substrate coated with more than three conductive layers cannot be manufactured using this process.